Existentialism is a movement in philosophy and literature that emphasizes individual existence, freedom and choice. It began in the mid-to-late 19th Century, but reached its peak in mid-20th Century France. It is based on the view that humans define their own meaning in life, and try to make rational decisions despite existing in an irrational universe. It focuses on the question of human existence, and the feeling that there is no purpose or explanation at the core of existence. It holds that, as there is no God or any other transcendent force, the only way to counter this nothingness (and hence to find meaning in life) is by embracing existence.

Thus, Existentialism believes that individuals are entirely free and must take personal responsibility for themselves (although with this responsibility comes angst, a profound anguish or dread), and emphasizes action, freedom and decision as fundamental in rising above the essentially absurd condition of humanity (which is characterized by suffering and inevitable death). For more details, see the section on the doctrine of Existentialism.

Existentialists refuse to belong to any school of thought, repudiating of the adequacy of any body of beliefs or systems, claiming them to be superficial, academic and remote from life. It is a reaction against traditional schools of philosophy, such as Rationalism, British Empiricism and Positivism, that seek to discover an ultimate order and universal meaning in metaphysical principles or in the structure of the observed world.

Both philosophers considered the role of making free choices on fundamental values and beliefs to be essential in the attempt to change the nature and identity of the chooser. In Kierkegaard's case, this results in the "knight of faith", who puts complete faith in himself and in God, as described in his 1843 work "Fear and Trembling". In Nietzsche's case, the much maligned "▄bermensch" (or "Superman") attains superiority and transcendence without resorting to the "other-worldliness" of Christianity, in his books "Thus Spake Zarathustra" (1885) and "Beyond Good and Evil" (1887).

The PhenomenologistMartin Heidegger was an important philosopher in the movement, especially his influential 1927 work "Being and Time", although he vehemently denied being an Existentialist in the Sartrean sense. Other major influences include Max Stirner (1806 - 1856), Karl Jaspers (1883 - 1969) and Edmund Husserl, and writers like the Russian Fyodor Dostoevsky (1821 - 1881) and the Czech Franz Kafka (1883 - 1924).

Existentialism came of age in the mid-20th Century, largely through the scholarly and fictional works of the French existentialists, Jean-Paul Sartre, Albert Camus (1913 - 1960) and Simone de Beauvoir (1908 - 1986), all of whose works popularized existential themes, such as dread, boredom, alienation, the absurd, freedom, commitment and nothingness. Maurice Merleau-Ponty (1908 - 1961) is another influential and often overlooked French Existentialist of the period.

Sartre is perhaps the most well-known, as well as one of the few to have actually accepted being called an "existentialist". "Being and Nothingness" (1943) is his most important work, and his novels and plays, including "Nausea" (1938) and "No Exit (1944), helped to popularize the movement.

In "The Myth of Sisyphus" (1942), Albert Camus uses the analogy of the Greek myth of Sisyphus (who is condemned for eternity to roll a rock up a hill, only to have it roll to the bottom again each time) to exemplify the pointlessness of existence, but shows that Sisyphus ultimately finds meaning and purpose in his task, simply by continually applying himself to it.

Simone de Beauvoir, an important existentialist who spent much of her life alongside Sartre, wrote about feminist and existential ethics in her works, including "The Second Sex" (1949) and "The Ethics of Ambiguity" (1947).